This invention relates to the purification of phosphoric acid by solvent extraction and, in particular, to the purification of wet process phosphoric acid.
In the so-called wet process, phosphate rock and sulfuric acid are reacted to form gypsum, hydrogen fluoride, and orthophosphoric acid, an example of such a process being described in Faith, Keyes and Clark Industrial Chemical, Third Edition, Wiley & Sons, 1965, pages 603-605. In many cases, the resultant phosphoric acid, often called green acid, must be purified in order to eliminate undesirable metallic impurities, especially when the phosphoric acid is used in the food and detergent industries. The impurities normally found in green acid are compounds of silicon, fluorine, calcium, arsenic, iron, aluminum, chromium, vanadium, lead and other metals in lower amounts.
It is old in the art to purify green acid by solvent extraction with an organic solvent which is either immiscible or partially miscible with water. In both cases, a two-phase system is obtained, the major portion of the phosphoric acid being dissolved in the organic phase, and the major portion of the impurities being dissolved in the aqueous phase. The two phases are then separated conventionally, by decantation, for example. In this connection, attention is invited in particular to German Patent No. 884,358, dated July 27, 1953.
Irrespective of the specific solvent extraction technique employed heretofore, a significant quantity of the phosphoric acid remained in the aqueous solution containing the impurities, the amount of phosphoric acid remaining being inversely proportional to the concentration of the green acid treated. Consequently, the yield of recovered P.sub.2 O.sub.5 decreased as the concentration of the treated acid decreased. For example, from green acids having a concentration of only 3 % of P.sub.2 O.sub.5, not more than 80 - 90 % of the P.sub.2 O.sub.5 can be recovered.
In an effort to circumvent the above disadvantage, it has been proposed to add sulfuric or hydrofluoric acid to the phosphoric acid. However, the added acid must then be separated from the phosphoric acid in a subsequent step, this being especially troublesome when the added acid is not volatile. It has also been suggested to add salts, such as sodium fluoride. However, these salts must be sufficiently pure, or else the impurities associated therewith can end up in the phosphoric acid product. Thus, if a salt is to be used, it must be recovered from the aqueous phase, purified, and then recycled, such steps resulting in an unduly complex and expensive process.